Hemodynamic monitoring, performed invasively with a pulmonary artery catheter, has become common in the care of the critically ill. Until recently, hemodynamic monitoring has been limited to the critical care unit, operating room and occasionally the emergency department, due to the invasive nature of the pulmonary artery catheter, the expertise required for insertion and maintenance of the catheter, and the close vigilance required to prevent potential vital risks to the patient. Accepted invasive hemodynamic monitoring methods include the Fick method, dye indicator dilution, and thermodilution.
Contact impedance cardiography systems now available provide a type of noninvasive monitoring of patient hemodynamics. Unlike invasive hemodynamic monitoring with a pulmonary artery catheter, noninvasive contact impedance monitoring is not restricted to care of the critically ill. Noninvasive continuous hemodynamic monitoring has utility in any clinical area, from the outpatient clinic to the critical care unit, where healthcare providers desire information regarding a patient's hemodynamic status without invasive procedure.
Conventional contact impedance cardiography operates by introducing an alternating current (AC) voltage test signal of low magnitude directly into the thorax. This is done using electrodes contacting the skin. The conductivity changes of the cardiac system, primarily blood flow through the heart, modulate the current introduced into the body by the voltage signal. The modulated current signal carries information that is compared to the original voltage test signal by a receiver/comparator to extract a cardiac impedance waveform. The direct current (DC) component of the modulated signal represents the fixed or baseline electrical impedance of the thorax, ZO and the AC component of the modulated signal represents the time varying cardiac impedance waveform ΔZ/Δt of the thorax. The conventional technology of contact impedance cardiography is disclosed and explained in U.S. Pat. Nos. 5,423,326, 5,443,073 and 5,309,917, incorporated by reference herein.
While contact impedance cardiography technology is a marked improvement over invasive technology, it still has some limitations. It requires the careful placement of several electrodes on the patient's torso. While this does not seem like a significant drawback, it has been an impediment to the movement of noninvasive continuous hemodynamic monitoring from the hospital emergency room to the medical first aid arena. Such medical first aid area encompasses virtually any emergency field situation including the military battle field, natural disaster, or other emergency medical scenario.
It has been discovered that the varying portions of a reflected radio interrogation signal can be processed in much the same way as the variations in the signals detected in conventional, contact impedance voltage measurements, to extract cardiac information. It will be appreciated that a reflected radio interrogation signal undergoes changes as a result of the encounters of the radio signal with different substances present in the human body. It has been found, in particular, that the reflected radio signal, like the much lower frequency impedance test signal conducted through the torso in conventional contact impedance measurement, is sensitive to electrically conductive substances and is modified in amplitude and phase at least in part by the dynamic changes of varying blood volume, flow velocity and possibly even alignment of the red blood cells that reflect the mechanical activity of the heart. The reflected radio interrogation signal, like the transmitted voltage test signal used in conventional, contact impedance measurement, has both a constant/baseline component (comparable to Zo) and a component that varies relatively slowly over time (100 Hertz or less) comparable to ΔZ/Δt, with at least first and second order components. Accordingly, like the transmitted test signal used in conventional, contact impedance measurement, the reflected radio interrogation signal carries information that can provide determination and even measurements of at least some of the same cardiac functions.